Typical present day optical communication systems use a glass transmission line, termed an optical fiber, to transmit an optical signal between a light source and a photodetector. For many applications, transmission of information in both directions is desired. Such systems are commonly termed bidirectional and often use two separate unidirectional transmission systems. That is, they use two optical fibers, two light sources and two photodetectors.
This configuration is frequently commercially undesirable because the added components, as compared to a unidirectional system, increase the cost of the system. Not only are two optical fibers required, but the light sources are typically semiconductor lasers which are relatively expensive at the present time. Additionally, many sites for at least one of the lasers would necessarily be on customer premises and might encounter hostile environments thereby degrading system performance. Accordingly, effort has been devoted to developing a bidirectional optical communications system architecture using fewer components.
For example, a bidirectional system has been demonstrated which uses a single optical fiber connecting the two users, i.e., terminals. A laser and photodetector are placed at one system terminal and a laser and photodetector are also placed at the other system terminal. The latter terminal will be termed the user site. While this architecture is perfectly adequate for many purposes, it is disadvantageous because it requires both a photodetector and a laser at both terminals. A system using still fewer components would be desirable.